An ultrasonic sensing equipment, which is mounted on a vehicle, such as an automobile, and used for detecting a distance between the vehicle and surrounding obstacles at the time of parking or turning thereof, is disclosed, for example, in Japanese Patent Publication No. 2001-16694. Such an ultrasonic sensing equipment is comprised of an ultrasonic element for emitting ultrasounds (ultrasonic waves) and another ultrasonic element for receiving them (or, in some cases, one ultrasonic element is commonly used as having the functions of both emitting and receiving the ultrasounds), wherein the ultrasounds are emitted by the emitting element and then the ultrasounds reflected by an obstacle are received by the receiving element. The sound pressure, time delay and phase difference of the ultrasonic waves received by the receiving element are used to detect a direction in which the obstacle exists and the distance to the obstacle, and/or to judge the degree of projections and depressions of the obstacle.
An ultrasonic sensing element, in which a vibrating device of a piezoelectric thin film is formed on a membrane which constitutes a thin film portion of a substrate, attracts attention recently as a receiving element to be used in the above ultrasonic sensing equipment for obstacle detection and/or other purposes. The ultrasonic sensing element of the membrane structure can be produced by means of semiconductor micromachining technology, which is hereinafter referred to as MEMS (Micro Electro Mechanical System) type ultrasonic sensing element. The MEMS type ultrasonic sensing element and an ultrasonic array sensor using the sensing elements, are disclosed, for example, in Japanese Patent Publication No. 2003-284182.
FIGS. 10A and 10B are simplified views of an ultrasonic element disclosed in the above Japanese Patent Publication No. 2003-284182, wherein FIG. 10A is a schematic top view of the ultrasonic sensing element 90 and FIG. 10B is a cross sectional view taken along a line XB-XB indicated by one-dot-chain line in FIG. 10A.
The ultrasonic element 90 shown in FIGS. 10A and 10B is made by using a semiconductor substrate 10 having the structure of SOI (Silicon On Insulator). In the substrate 10, reference numerals 1a, 1b, 1c and 1d respectively designate a first semiconductor layer (a supporting substrate), a buried oxide film, a second semiconductor layer and a protection oxide film. A piezoelectric vibrator 20 is formed on a membrane M, which is formed as a thin film portion of the substrate 10 by semiconductor micromachining technology, so as to wholly cover the membrane M. The piezoelectric vibrator 20 has a structure in which a piezoelectric thin film 2 is sandwiched between metal electrode films 3a and 3b. The ultrasonic element 90, as shown in FIGS. 10A and 10B, receives the ultrasounds reflected at the obstacle as a result that the piezoelectric vibrator 20 as well as the membrane M is resonated with the ultrasounds at a prescribed ultrasonic wave-band frequency.
The MEMS type ultrasonic element 90 shown in FIGS. 10A and 10B can be manufactured in tiny size and at low cost, even in case a plurality of ultrasonic elements are arrayed on the substrate 10, and furthermore the array configuration makes it possible to perform not only distance measurement but two-dimensional (2D) and three-dimensional (3D) measurement. Therefore, an ultrasonic array sensor employing such MEMS type ultrasonic element 90 as shown in FIGS. 10A and 10B has been in a development.
On the other hand, the MEMS type ultrasonic element 90 shown in FIGS. 10A and 10B is an ultrasonic element for an exclusive use of receiving, and consequently unusable at present for emitting purpose due to difficulty in outputting ultrasonic waves of sufficient sound pressure. The detection capability of 1.5 to 3 meters is generally required in the obstacle detection for automotive use, wherein the ultrasonic waves are spread in the air within the round distance of 3 to 6 meters. Since the attenuation of ultrasonic waves is higher in the air, the S/N ratio is decreased when receiving the reflected ultrasounds. Therefore, it is practically impossible to detect an obstacle, without enhancing the sound pressure of ultrasounds to be emitted. For these reasons, it is difficult to employ the same structure to the ultrasonic element 90 shown in FIGS. 10A and 10B, as an ultrasonic element for emitting the ultrasounds. In fact, as disclosed in Japanese Patent Publication No. 2003-28418, a large-sized ultrasonic emitting element (device) is separately installed to form an ultrasonic sensing equipment. According to such a separate installation of the emitting element and the receiving element, as in Japanese Patent Publication No. 2003-28418, product cost will be increased and similarly weight increase as well as design inferiority may occur due to the larger configuration.